1. Field of the Invention
The present invention relates to a strut that can be locked by an external power source to assist in termination of further motion of the strut, particularly for use in a door.
2. Description of Related Art
Motor vehicle liftgates act to both provide access and close and seal the rear cargo area of a motor vehicle. Typically, the liftgate is pivotally mounted in a frame at the rear of the vehicle and pivots about a hinge defining a horizontal axis. The liftgate rotates between a closed position securedly resting within the frame and an open position, wherein the liftgate is pivoted away from the frame to allow access to the cargo area. The liftgate is often very heavy and must be moved against gravity to reach an open position. Access to the cargo area is difficult and dangerous when a user is required to lift the liftgate to the open position unassisted, and then hold the liftgate in position while accessing the cargo area.
Most modern vehicles use gas or spring-loaded cylindrical struts to assist the user while opening the liftgate, and then to hold the liftgate in an open position. Typically, the user manually provides the initial force necessary to partially open the liftgate. The strut then provides a spring force and a moment arm sufficient to overcome the weight of the liftgate and move the liftgate to a fully opened position. The spring force and the moment arm of the strut then act to hold the liftgate in the open position while the user accesses the cargo area. To return the liftgate to a closed position, the user must typically thrust downward on the liftgate, applying a force sufficient to overcome the upward forces exerted by the strut. Typically a liftgate assembly includes two struts at opposite ends of the frame. One end of each strut is pivotally mounted to the liftgate, while the other end of each strut is pivotally mounted to either the frame or the motor vehicle.
Powered systems for automatically moving vehicle liftgates between an open and a closed position are also known in the art. Typically in such systems, a power actuator applies a force directly to the liftgate. For example, U.S. Pat. No. 5,531,498 to Kowall discloses a typical liftgate opening system wherein the struts are actuated by a pair of cables wound around a spool by an electric motor, replacing the user-supplied force necessary to initiate movement of the liftgate. However, a significant amount of power is required to operate such a system, resulting in a mechanism that is usually quite large and uses a significant amount of vehicle space.
Another example of a powered liftgate system is illustrated in U.S. Pat. No. 6,367,864 to Rogers, Jr. et al. This system provides a rod in addition to the struts. The rod is pivotally mounted to a follower mounted on a fixed linear channel. A flexible drive loop moves the follower to drive the liftgate between open and closed positions. Since the liftgate is directly connected to the drive, some form of clutch or disengagement mechanism is required to allow manual operation of the liftgate. This disengagement mechanism further consumes vehicle space while also increasing costs.
U.S. Pat. No. 5,120,030 to Lin et al. provides yet another example of a powered liftgate system. A magnet is provided on a piston to better retain the piston in a fully extended position. The force exerted by the magnet acts with the force generated by the strut to increase the force required to initiate compression of the strut when moving the door out of an open position. For example, with the strut fully extended, a force between 600 N at a strut temperature of −40° C. and 800 N at a strut temperature of +85° C. is required to initiate collapse of the strut with the magnetic force disabled. When the magnet is engaged, a considerably higher initial force is required to overcome the magnetic force. After this initial force is applied and the magnetic force is overcome, the strut collapses normally. The disadvantage with this type of system is that the separation of the magnet may require a rising force followed by a sudden release, causing the liftgate to lurch at the point of release.
Control systems for powered liftgate systems are also known in the art. Such control systems usually include an obstacle detection component to stop the liftgate while opening or closing if an obstacle is encountered. Typically the control system measures the force applied by the liftgate or the actuator motor, or the rate at which the motor is moving. The liftgate is stopped if anomalies are detected in the measurements tending to indicate that an obstacle has been encountered.
Finally, a large engagement force is necessary to activate most prior art locking struts, which typically include a driver outside of the strut housing. In this type of system, a rod must travel down the center of a piston rod to translate the signal from the driver to a valve. A significant pressure differential exists between the outside atmospheric pressure where the driver is located and the pressure within the housing. To resist this pressure differential, the driver must exert a significant engagement force to activate the valve. Therefore, it is desirable to provide a system wherein the driver is housed within the housing, thereby reducing or eliminating the pressure differential and requiring much lower engagement forces.
It is desirable to provide a a locking strut or struts to smoothly open and close the liftgate and lock the strut in a fully extended position without adding unnecessary bulk or cost to the vehicle.
Additionally, it is desirable to provide a driver located within a strut housing to significantly decrease the pressure differential between the driver and a valve. The strut is connected to a control system to provide for control of the strut during movement between open and closed positions.